1. Field of the Invention
This invention relates generally to a web tracking apparatus, and more specifically to the tracking of a flexible web moving in a closed loop path defined by two or more hard surface cylindrical supports.
2. Description of the Prior Art
A web tracking apparatus for tracking a flexible, unidirectionally moving web in a closed loop path on hard surface, cylindrical web supports comprise basically two types of web supports if defined in terms of function rather than structure. That is, as the moving web approaches a web support, the entering web "sees" the support, relative to a fixed frame, either as (1) a laterally constraining support, or (2) a laterally non constraining support. A laterally constraining support may be further subdivided into (a) an angular lateral constraint in which the entering web is constrained against changing its lateral position relative to the frame except as its angular position changes relative to the fixed frame, and (b) a positional lateral constraint in which the entering web is constrained against changing its spatial lateral position relative to the frame, while remaining free to change its angular position. The web entering a non constraining support, on the other hand, is free to change either its angular or its spatial lateral position relative to the frame without experiencing substantial lateral forces.
Whether a particular web support is a laterally constraining or laterally non constraining support depends as much on its function in the tracking apparatus as on its structure. For example, a fixed axis, rotating, cylindrical roller, such as an idler roller or a drive roller in a tracking apparatus, is structurally an angular lateral constraint capable of constraining the moving web against change in its lateral position. To perform as an angular lateral constraint, the entering web has to be capable of tracking on the rotating cylindrical surface until the moving web and the rotating surface are in alignment; i.e., until the longitudinal axis of the rotating surface is perpendicular to the direction of travel of the web. This tracking phenomenon is due to frictional forces developed between the moving web and the rotating surface, which in turn are a function of, among other variables, wrap angle, web tension, and the upstream web-span to web-width ratio. Thus, if the wrap angle, for example, is insufficient to create the frictional forces necessary for tracking, the entering web is free to change its angular position and/or its lateral spatial position, without experiencing substantial lateral forces resulting in a web support that is functionally non constraining, although structurally an angular lateral constraint.
Although the above-noted variables upon which tracking depend are usually parameters which are governed by the design of the web tracking apparatus, some generalities can be stated that cover a significant number of situations. Thus, for a flexible closed loop web supported by hard surface cylindrical supports, the upstream web-span to web-width ratio should be somewhat equal to or greater than one, and the wrap angle should range between aproximately 30.degree. and 135.degree., depending on the coefficient of friction of the surfaces in contact, and on web tension. If otherwise, the web could be prevented from tracking, either because of not enougg, or too much contact with the web support.
To facilitate the discussion to follow, it will be convenient to refer to a laterally non constraining support, as defined above, as an "N" support, and to refer to a laterally constraining support as a "P" support if it is functionally a positional lateral constraint as defined above, and as an "A" support if it is functionally an angular lateral constraint, also as defined above.
In designing a closed loop web tracking apparatus of the type discussed above, one of the primary considerations of the design is lateral stability of the moving web. Generally, stability of the moving web is achieved if the tracking apparatus has at least two laterally constraining supports, at least one of which is further restricted to be a P support; the remaining web supports, if any, in the tracking apparatus can be either laterally constraining supports (P or A) or non constraining supports (N) as dictated by design considerations.
Although the stability principle stated above will ensure stability of the moving web, it does not, without more, ensure uniformity of tension in the moving web. Nonuniformity in tension ordinarily results from imperfections in the manufacture of webs and web supports, and from the lack of perfect parallelism in the longitudinal axes of the mounted web supports. It follows that if manufacturing tolerances are minimized and the supports are mounted with a high degree of parallelism, a degree of uniformity of tension will be achieved. However, such considerations are independent of the stability principle.
If a high degree of uniformity of tension of the web is a requisite of the tracking apparatus design, it can be achieved with little regard to manufacturing or mounting tolerances by conforming the web tracking apparatus to what will be referred to as the uniformity of tension principle. This second web tracking principle dictates that the moving web exiting from a first laterally constraining support must be given freedom, once and only once, to change direction before entering a second laterally constraining support. This freedom is given to the exiting web by "gimballing" the first web support; i.e., by mounting the first web support for pivotal movement about a gimbal axis which is parallel to the direction of linear movement of the entering web, and which intersects the longitudinal axis of the support at the midpoint of the support. If for design considerations it is impracticable to gimbal the first constraining web support, e.g., if it is a drive roller, the uniformity of tension principle can be met by gimballing a non constraining web support mounted between the two constraining web supports.
The gimbal action of the web support, i.e., the capability of the exiting web to change direction, enables the exiting web to compensate for non uniformity of tension of the web in the downstream web span. The resultant force of the non uniform tension across the exiting web is at some perpendicular distance from the centerline of the moving web; the component of that resultant force which is perpendicular to the gimbal axis creates a moment about the gimbal axis which varies with the sine of the wrap angle, since the magnitude of the force component perpendicular to the gimbal axis varies with the sine of the wrap angle. For example, for wrap angles approaching zero or 180.degree. the magnitude of the force component approaches zero and therefore, the exiting web is not free to change direction.
It is clear from the above relationship that the magnitude of the force component perpendicular to the gimbal axis is greatest for a wrap angle of 90.degree.; moreover, as the wrap angle increases appreciably from 180.degree. the exiting web behaves as if the wrap angle were appreciably greater than zero. While the gimbal action may not be appreciably inhibited by large wrap angles (e.g., those appreciably less than 180.degree. and especially those appreciably greater than 180.degree.), such large wrap angles may inhibit the tracking action of a web support, thereby possibly producing an unstable tracking apparatus.
The "once and only once" requirement of the uniformity of tension principle can be illustrated by theorizing a tracking apparatus in which the web exiting from a P or A support encounters two N supports before entering a second laterally constraining support. The "once and only once" requirement provides that only one of the three supports encountered by the moving web before reaching the second constraining web support (the first laterally constraining P or A support, the first N support, or the second N support,) be gimballed; the other two must prevent the exiting web from changing direction. For reasons noted above, gimballing one of the supports provides uniformity of tension in the downstream web without affecting lateral stability. However, if more than one support is gimballed before the web enters a second lateral constraint, the lateral position of the web at the second and any subsequent non constraining gimballed supports, becomes unstable and indeterminate. The result could be lateral instability of the web span between the first gimballed support and the second constraining web support, and possible edge damage to the moving web due to such instability. Thus, the "once and only once" requirement ensures lateral stability in the moving web when N supports are utilized in a tracking apparatus, while providing uniformity of tension.
Theoretically, the above principles would not be violated by a two-support, closed loop web tracking apparatus. However, technical problems such as the gimballing of a drive roller to meet the "once and only once" requirement, and wrap angle considerations upon which the gimballing action depends, as well as practical problems such as utility for such a two-support apparatus, could make such an apparatus commercially unattractive. On the other hand, if uniformity of tension in the web is not crucial in the design of the tracking apparatus, a tracking apparatus comprising two laterally constraining supports, at least one of which is a P support in conformance with the stability principle outlined above, will provide a laterally stable web tracking apparatus. A measure of uniformity of tension in the web will be maintained if manufacturing and mounting tolerances are kept at a minimum, notwithstanding the violation of the "once and only once" principle outlined above. In such an apparatus, gimballing the two supports would not provide the necessary gimballing action since the wrap angle of one or both laterally constraining supports would be, in practice, about 180.degree.. The introduction of additional supports to a closed loop web tracking apparatus, however, eliminates the technical and practical problems discussed above if the combination of supports conforms to the two tracking principles outlined above, and their location relative to each other is such that the respective wrap angles the moving web makes with the three or more supports are within the limits previously discussed.
Structurally, the art discloses a variety of individual web supports which are the functional equivalent of the basic supports outlined above. For example, the art discloses laterally constraining supports of the P type which can be placed in two categories: (1) "active" P supports, and (2) "passive" P supports. The active category includes servo controlled steering rollers in which lateral position of the web is maintained by an external mechanism which senses the misalignment of the web and triggers a compensating mechanism to return the moving web to its aligned position. The external sensing mechanism can be mechanical, electrical, or pneumatic and generally has the economic disadvantage of being complex and expensive. There is also the added practical disadvantage of lateral web oscillation, however tolerable, between the aligned and the sensed misaligned position.
The passive category of P supports includes castered and gimballed rollers having edge guides which are fixed in a predetermined position relative to a fixed frame, and are fixed a distance apart which corresponds substantially to the width of the moving web. Hence, the edge guides fix the lateral spatial position of the moving web at the roller.
Non constraining N type supports disclosed by the art include low friction cylindrical non rotating surfaces, axially compliant rollers, as well as cylindrical rollers mounted for pivotal movement about a castering axis which is perpendicular to the plane of the entering web and which intersects the centerline of the moving web at an upstream location, such as that disclosed in copending application Ser. No. 504,777. The non constraining action of a castered roller is dependent upon the tracking phenomenon discussed above in connection with a fixed axis, angularly constraining roller, and is dependent upon the same variables (i.e., wrap angle, etc.). In the case of the angular laterally constraining support the web aligns itself perpendicular to the fixed axis, and thereby its lateral position becomes fixed. However, in the case of the non constraining castered roller, the roller aligns itself to the moving web, thereby giving the web freedom to change either its angular or spatial lateral position.
The art discloses closed-loop web tracking apparatus of hard surface cylindrical rollers supporting flexible moving webs. Such tracking apparatus depend on servo controlled steering rollers to provide the necessary lateral control of the moving web and, as such, may be called active web tracking apparatus. Besides the servo controlled P support, these unidirectional devices also include a drive roller by which longitudinal movement in the closed loop path is imparted to the closed loop web, and which constrains the moving web angularly against change in its lateral position. It is seen, then, that active tracking apparatus can conform to the stability principle of web tracking. As previously noted, however, these devices have economic and practical disadvantages. Such disadvantages can be overcome by a passive P support, which, as used herein, is defined as an apparatus free of misalignment sensing and compensating mechanisms, such as a servo controlled steering roller.
Accordingly, it is an object of the invention to provide a web tracking system for the accurate tracking of a flexible, closed-loop web that is mechanically simple and reliable, and independent of external misalignment sensing mechanisms.
It is another object of the invention to provide a web tracking system wherein the tension distribution in the closed-loop web is uniform and the moving web is positionally stable.
It is a further object of the invention to provide a web tracking apparatus that is self compensating for variations in web dimensions and stiffness, and parallelism tolerances in the longitudinal axes of the web supports.